Grazing limitation and nutrient limitation in marine ecosystems: Steady state solutions of an ecosystem model with multiple food chains

Here I develop a model with multiple phytoplankton-zooplankton food chains where each food chain is based on a different size class of algae. Model parameters were chosen to allow food chains based on smaller algal size classes to dominate under oligotrophic conditions, with larger size classes being added sequentially with increased nutrient loading. The model makes use of allometric relations among size classes to minimize the number of free parameters, facilitating numerical exploration of its steady state behavior. The model is simple, yet it is complex enough to allow simultaneous predator limitation of algal size classes (or species) and nutrient limitation of total phytoplankton biomass. This perspective, not obtainable from models with single phytoplankton and zooplankton size classes, is applied to the particular case of elevated nitrate levels in the equatorial Pacific. The model shows clearly that it is possible for each phytoplankton size class to be limited by its herbivores, while at the same time micronutrient (notably iron) deficiency may limit the number of size classes that can exist in the community and hence the total phytoplankton biomass that can be supported.